Magnetic recorders can read and write information on magnetic tape. The presence of physical defects on the surface of the tape can cause the recorder to misread the information on the tape, resulting in a reading "error". Such physical defects typically include scratches on the surface of the tape, crinkles in the tape itself, or the presence of large particles on the tape.
It is known that optical detection devices can be used to detect physical defects in magnetic tape. Such devices can be located on the same side of the tape as a light source so as to detect light that is reflected at the surface of the tape. In the alternative, the detection device can be located on the opposite side of the tape from the light source so as to detect light which passes through the tape. In either case, light which contacts a flat portion of the tape surface will be reflected/transmitted toward detectors in the detection device. Light which strikes a physical defect on the surface of the tape will be scattered and thus will not be detected by the detectors. Whenever one of the detectors fails to receive enough light, an indicator light is lit to indicate that the tape is defective.
Detection devices employing various light source and detector arrays have been used to detect physical defects on the surface of magnetic tape. One array includes a single row of detectors spanning the width of the tape. Another array uses approximately 24 detectors in a two-dimensional array where the detectors are aligned in several columns which are not quite parallel with the direction of travel of the tape. Another method of detecting tape surface defects is to utilize a row or array of fiber optic strands to receive the light and convey it to the detectors.
The optical detection device must be able to detect all physical defects of a certain size, regardless of their location on the tape. If too few detectors are used, there is the risk that physical defects on the tape may be missed. The use of too many detectors, however, needlessly increases the cost of the device.
It would be desirable to have an optical detection device for screening for defects in magnetic tape which uses a minimum number of detectors and yet can reliably detect all physical defects of a particular size regardless of their location on the tape surface.